Ultrasonic sensor for controlling the process during resistance spot welding

ABSTRACT

A sensor carrier is provided that has at least one oscillating element ( 32 ) that introduces ultrasonic waves into an area to be examined and/or receives ultrasonic waves coming from the area to be examined, whereby the oscillating element ( 32 ) is located in an oscillator carrier ( 33 ), and the oscillator carrier ( 33 ) has connecting means ( 36, 37, 38, 39 ) for connection with the electrode ( 31 ).

Background Information

[0001] The present invention relates to an ultrasonic sensor forcontrolling the process during resistance spot welding according to thegeneral class of the independent claim.

[0002] The essence of the method described in European PatentApplication EP-A-653 061 is to investigate the intended weld region byultrasonic transmission during the welding operation using shear andtransversal waves by positioning an ultrasonic transmitter and anultrasonic receiver for shear waves on each of the external electrodeadapters of the two diametrically opposed welding electrodes. Startingat the ultrasonic transmitter on one welding electrode, the ultrasonicsignal passes through the weld material—two or more sheets to bewelded—and the other welding electrode until it reaches the ultrasonicreceiver. Said ultrasonic receiver converts said ultrasonic signal to ameasurable electrical signal U, the shape of the curve of which overtime can be depicted using the equation U=U_(O)·sinωt, where ω=2 πf isthe angular frequency of the ultrasonic wave, f is the frequency, and tis the time. The through-transmitted signal is detected online, and itsamplitude U_(O) is used as the control variable for amplitude and theshape of the welding current curve over time. The transversal wave isselected because the influence of fluid formation in the weld nugget onthe dampening of a through-transmitted wave is very strong with thistype of wave. The amplitude U_(O) of the transversal wave, which changesmarkedly and in characteristic fashion over the course of the weldingprocess, permits a reliable determination of the formation and size ofthe weld nugget and can therefore be used as a manipulated variable fora control process.

[0003] The basic feasibility of the method and the reliability of theexamination findings are crucially dependent on the ultrasonic sensorsused, their location relative to the welding electrodes, and the soundpropagation inside the welding electrodes. In the realization accordingto EP-A-653 061, an arrangement of ultrasonic sensors is selected inwhich the ultrasonic transmitter and ultrasonic receiver are mounted onthe external electrode adapters or on the electrode holders, which arenot shown in the drawing. Shear waves, transversal waves, or torsionalwaves having a frequency of less than 1 MHz are generated. It is statedthat it is particularly advantageous to generate horizontally polarizedtransversal waves, since they have a low tendency to undergo undesiredmode changes when reflections occur inside the sound-directingelectroder holder. The ultrasonic transmitters and receivers are “shearwave test heads”. They contain flat and, usually, round piezoelectricplates having a diameter ranging from a few mm to a few cm, and thatexecute a shearing motion when excited with electric voltage or,conversely, when they receive, they react to a received shear wave witha reception voltage. Since, when a shear wave test head of this type ismounted directly on the external electrode adapter, the main emissiondirection of the sound would not be directed in the direction of theweld material, but rather at the center of the electrode, wedge-shapedattachments are preferably used, that are installed between the testheads and the welding electrodes and permit the main emission directionof the test head to be aligned with the weld material at an angle thatis markedly less than 90°, e.g., approximately 45°. This arrangementdoes now allow all of the sound energy that is produced to beconcentrated in the direction of the welding spot, however.

[0004] German Patent Application DE-A-199 37 479, which was published ata later date, describes an ultrasonic sensor system that is improved inthis regard. With said ultrasonic sensor system, the piezoelectric shearwave plate or a complete shear wave test head is installed in a recessinside the electrode adapter for transmitting and/or receiving. In fact,said piezoelectric shear wave plate or the complete shear wave test headis installed in such a manner that the piezoelectric plate is orientedalmost perpendicular to the electrode adapter, and the main emissiondirection of the transmitter and the main reception direction of thereceiver are therefore parallel to the electrode adapter and aredirected toward with each other. This allows all of the ultrasonicenergy that is produced to be concentrated in the direction of thewelding spot and, from there, it is transmitted in the direction of thereceiver. This allows such a level of ultrasonic intensity to beproduced in the welding spot and, during reception, it allows a receivedsignal to be generated that is so great that a much greaterwanted-to-unwanted signal ratio exists with regard for the furtherevaluation for controlling the welding process.

[0005] In the case of the latter sensor system, described in GermanPatent Application DE-A-199 37 479, in which the sensors are integratedin specially-configured welding electrodes and that is optimized withregard for the wanted-to-unwanted signal ratio of the ultrasonicsignals, it is also considered disadvantageous that the electrodeadapters typically used with spot welding can no longer be used. It waspossible to manufacture said electrode adapters as a simple turnedcomponent at very low cost, due to their smooth cylindrical shape. Withthe sensor system according to German Patent Application DE-A-199 37479, however, the complete welding electrode with integrated sensor mustbe replaced if mechanical damage occurs to the electrode adapter or if asensor defect occurs.

[0006] With resistance spot welding, however, a large number of weldobjects have different shapes, so a large number of welding electrodeshaving different shapes is used. Since the diameter of the weldingelectrodes is the same for most applications, the large variety ofdifferent types is due mainly to the welding electrodes having differentlengths and the end pieces having different shapes, which said endpieces must be accommodated by corresponding electrode holders. Thereare also many different types of said electrode holders. For thisreason, one must put up with a large variety of different types ofwelding electrodes having integrated sensors with regard for theacoustically optimized sensor system according to German PatentApplication DE-A-199 37 479 as well.

[0007] With regard for the sensors mounted on the external electrodeadapter, no statements are made in European Patent Application EP-A-653061 about how the shear wave test heads, including the wedge-shapedattachments, are mounted on the external electrode adapter. This is afundamental problem. Not only must the connection have good acousticconductivity, it must also be so mechanically sound that the sensorsand/or wedge-shaped attachments are able to withstand the highaccelerations that occur when the welding tongs open and close duringthe welding process and not drop out of the welding electrodes when thewelding electrodes impact the sheets to be welded. Since, in addition,considerable temperature fluctuations can occur at the weldingelectrodes, an adhesive connection is not suitable. This would alsoprevent easy replacement of the sensors.

[0008] The object of the present invention, therefore, is to provide anultrasonic sensor for controlling the process during resistance spotwelding, in which the sensors and test heads are oriented entirely inthe direction of the welding spot and are mounted on the externaladapter of preferably cylindrically shaped welding electrodes in such amanner that simple installation and replacement is enabled.

ADVANTAGES OF THE INVENTION

[0009] The sensor carrier according to the invention has at least oneoscillator element that introduces ultrasonic waves into an area to beexamined and/or receives the ultrasonic waves coming from the area to beexamined, whereby the oscillator element is located in an oscillatorcarrier, and the oscillator carrier includes connecting means forconnection with an electrode or electrode adapter of a welding system.The connecting means ensure, in simple fashion, that the complete sensorsystem need not be replaced when the welding electrode becomes worn. Theconnecting means are therefore designed to be detachable, so that, whenthe welding electrodes are replaced, the oscillator carrier is firstremoved from the electrode or the electrode adapter and then reinstalledin the new electrode. The sensor carrier according to the inventionfurther makes it possible to use conventional, commercially availableelectrodes for the resistance welding system to be used, which saidelectrodes are subsequently equipped with the sensor carrier.

[0010] In an advantageous further development, the sensor carrier iscapable of being joined with the electrode in a positively-interlockingmanner. As a result, the ultrasonic waves—which are preferablyconfigured as shear waves—propagate easily from the oscillator carrierthrough the electrode to the area to be investigated by ultrasonictransmission. An advantageous embodiment further provides that theoscillator carrier is designed annular in shape, with an inner diameterthat corresponds to the outer diameter of the welding electrode with anexact fit. In addition, the oscillator carrier has a slotted design,whereby the connecting means are designed as a clamping device that, inthe clamped state, establish a bonded connection between the outersurface of the welding electrode and the inner surface of the oscillatorcarrier in order to transfer ultrasonic waves. The oscillator element isso positioned that its face normal is oriented nearly parallel to thecentral axis of the annular oscillator carrier, or it forms an anglewith said central axis that does not exceed 15 degrees. This ensuresthat the ultrasonic waves are certain to reach the area to beinvestigated. The advantage of the proposed arrangement is its simplehandling in terms of replacing the sensor carrier. Said proposedarrangement also ensures that the ultrasonic waves are certain to reachthe electrode and the area to be investigated. With the clamping devicethat has a gap, it is also ensured that the sensor carrier is capable ofbeing mounted on electrodes having a slightly deviating diameter withouthaving to provide different sensor carriers in each case for differentelectrodes.

[0011] In an advantageous further development, it is provided that theacoustic impedance of the material of the oscillator carrier is similarto that of the material of the welding electrode. As a result, theultrasonic transmission from the oscillator carrier to the weldingelectrode can be further improved.

[0012] Further advantageous developments result from the furtherdependent claims and the description.

DRAWING

[0013] An exemplary embodiment is shown in the drawing and is describedin greater detail hereinbelow.

[0014] Using a simple exemplary embodiment of a sensor for controllingthe process during resistance spot welding, FIG. 1 shows the basicdesign and mode of operation of the present invention. FIG. 1 a is asectional drawing through the welding electrode with the sensor mountedthereon. FIG. 1b is a top view of the sensor arrangement according toFIG. 1a.

[0015]FIG. 2 shows the sketched wave fronts of the sound emission addedto FIG. 1a.

[0016] In particular, the invention utilizes the physical effect that alow-frequency (<1 MHz) shear wave—as proposed according to EuropeanPatent Application EP-A653 061 for controlling the process duringresistance spot welding, or, in general, any other waveform of acorrespondingly low-frequency ultrasonic wave that is introduced into awelding electrode that is cylindrical and hollow inside in order toaccommodate cooling water—propagates more or less homogeneously throughthe entire cross section of the welding electrode on its way to thereceiver on the other welding electrode. This is due to the fact that,below a frequency of 1 MHz, at typical propagation speeds of 3000 m/s,the wavelength of the shear wave in the cylindrical shaft of the weldingelectrode ranges from a few millimeters to a few centimeters. Weldingelectrodes typically have an outer diameter of 15-30 mm, and their wallsare typically 4-8 mm thick. The cross section of the electrode adapteris therefore of an equal or smaller order of magnitude than thewavelength. The cross section of the welding electrode itself is alreadysuch a small aperture opening for the propagating ultrasonic wave that alargely undirectional propagation of sound takes place, and the soundwave fills the entire cross section of the electrode adapter after justa short path of travel.

[0017] An annular oscillator carrier 33 is located around thecylindrical shaft of the welding electrode 31. The inner diameter of theannular oscillator carrier 33 is so selected that it is oversized onlyslightly compared to the outer diameter of the welding electrode 31,enabling the oscillator carrier 33 to be slid, with an exact fit, ontothe shaft of the welding electrode 31 and to be removed just as easily.The oscillator carrier 33 is slotted on one side. A narrow gap 38 islocated at the slotted point. Material recesses 35.1 and 35.2, a throughhole 36 and a threaded hole 37 are formed in the oscillator carrier 33to the left and right of the gap 38, enabling the two legs of theoscillator carrier 33 to be pulled together when the screw 39 is screwedinto the threaded hole 37, and the width of the gap 38 is reduced. Oncethe oscillator carrier 33 is slid onto the welding electrode 31, thescrew 39 located in the thread 37 is tightened to clamp the oscillatorcarrier 33 tightly and flush with the welding electrode 31. Depending onthe draw-in force of the screw 39, the inner surface of the oscillatorcarrier 33 and the outer surface of the welding electrode 31 then form amore or less bonded connection with each other, across which alow-frequency ultrasonic wave can be easily transmitted, even withoutinstalling coupling means.

[0018] A further material recess 35.3 is located in the oscillatorcarrier 33, in which a piezoelectric oscillator element 32 or a completeultrasound test head is inserted. In FIG. 1, rectangular piezoelectricoscillator elements are used. Basically speaking, however, saidoscillator elements can have another geometric form (e.g., round,semicircular, or rhombic) as well.

[0019] Oscillator element 32 is so positioned in oscillator carrier 33that the face normal of oscillator carrier 33—which is identical to themain emission direction—extends parallel to central axis 34, so that,during transmission, the sound is emitted in the direction of thewelding spot. At low frequencies (<1 MHz) and with oscillator dimensionsthat are not too great (e.g., <20 mm), the wavelength of the ultrasoundthat is produced is of the same order of magnitude as the edge length ofthe oscillator element 32, or it is even greater than the latter. Thisis why the sound emission is virtually undirectional and spherical, asindicated in the sketched wave fronts 45 in FIG. 2. The entireultrasonic wave generated by the piezoelectric oscillator element 32therefore travels from the electrode adapter 31 to the oscillatorcarrier 33 after just a short path of travel.

[0020] During reception, the same considerations basically apply for alow-frequency ultrasonic wave emitted from the base 46 with regard forthe spacial propagation of sound in the welding electrode 31 and theoscillator carrier 33. For this reason, the sensor system described canbe used as a transmitter and a receiver: due to the large wavelength ofthe sound wave and the small dimensions of the welding spot and/or thebase 46 of the welding electrode 31, this cross section—as well as anyother cross section—of the welding electrode 31 forms a small apertureopening, so that an almost spherical propagation of sound would takeplace without any lateral material restrictions. During reception,therefore, a greater portion of the sound energy also reaches theoscillator element 32. Since its surface is also oriented almostparallel to the wave front, a very high reception voltage can be pickedoff between the top side and bottom side of the piezoelectric element.

[0021] In order to optimize the alignment of the oscillator element 32with the wave front, said oscillator element 32 can be tilted slightly,so that the face normals of the oscillator element 32 and the axis 41form an angle having few angular degrees. In order to optimize thewaveform of oscillation, the piezoelectric oscillator elements 32 can beequipped with a damping element on the back side.

[0022] In addition, acoustic adaptive layers can be located between theoscillator carrier and the piezoelectric oscillating elements.

[0023] In order to prevent acoustic reflections between oscillatorcarrier 33 and welding electrode 31 during sound transmission, amaterial can be selected that has an acoustic impedance (the product ofdensity and sound propagation velocity) that corresponds to that of thematerial used to make the welding electrodes 31.

[0024] Refractive effects are prevented by selecting a material for theoscillator carrier 33 having a sound propagation velocity thatcorresponds to that of the material used to make the welding electrodes31.

[0025] Piezoelectric transducers can also be designed and manufacturedhaving a stacked configuration. This technique can be usedadvantageously with regard for the present invention. When n elementsare electrically connected in parallel, it is possible to generateacoustic amplitudes that are n-fold higher during transmission, forinstance, and to generate quantities of charge that are n-fold greaterduring reception, with excitation voltage remaining the same.

What is claimed is:
 1. A sensor carrier having at least one oscillatingelement (32) that introduces ultrasonic waves into an area to beexamined and/or receives ultrasonic waves coming from the area to beexamined, whereby the oscillating element (32) is located at or in anoscillator carrier (33), wherein the oscillator carrier (33) hasconnecting means (36, 37, 38, 39) for connection with the electrode(31).
 2. The device as recited in claim 1, wherein the connecting means(36, 37, 38, 39) establish a detachable connection between theoscillator carrier (33) and the welding electrode (31).
 3. The device asrecited in one of the preceding claims, wherein the oscillator carrier(33) is designed positively-interlocking with the electrode (31).
 4. Thedevice as recited in one of the preceding claims, wherein the oscillatorcarrier (33) is designed annular in shape in such a manner that itsinner diameter approximately corresponds to the outer diameter of thewelding electrode (31).
 5. The device as recited in one of the precedingclaims, wherein the connecting means (36, 37, 38, 39) are designed as aclamping device.
 6. The device as recited in one of the precedingclaims, wherein the oscillator carrier (33) has a slit (38) in thelongitudinal direction, said slit being changeable by the connectingmeans (36, 37, 38, 39) in order to produce a positively-interlockingconnection with the electrode (31).
 7. The device as recited in one ofthe preceding claims, wherein the oscillating element (32) is sopositioned that its face normal is oriented almost parallel to thecentral axis of the oscillator carrier (33), or it forms an angle withsaid central axis that does not exceed 15 degrees.
 8. The device asrecited in one of the preceding claims, wherein the oscillating elements(32) that are used are designed as shear wave plates.
 9. The device asrecited in one of the preceding claims, wherein the oscillating element(32) has a damping element on the back side.
 10. The device as recitedin one of the preceding claims, wherein acoustic adaptive layers arelocated between the oscillating elements (32) and the oscillator carrier(33).
 11. The device as recited in one of the preceding claims, whereinthe acoustic impedance of the material of the oscillator carrier (33) issimilar to that of the material of the welding electrode (31).
 12. Thedevice as recited in one of the preceding claims, wherein the soundpropagation velocity of the material of the oscillator carrier (33) issimilar to that of the material of the welding electrode (31).
 13. Thedevice as recited in one of the preceding claims, wherein the frequencyof the oscillating element (32) is less than 1 MHz.
 14. The device asrecited in one of the preceding claims, wherein the oscillating element(32) has a stacked configuration and is composed of at least twopiezoelectric plates or disks that are stacked one on top of the other,in alignment with each other.